The present invention relates to communications equipment, and, more particularly, to blind equalization in a receiver.
In blind equalization, the adaptive filters of a receiver are converged without the use of a training signal. As known in the art, there are two techniques for blind equalization: one is referred to herein as the xe2x80x9creduced constellation algorithmxe2x80x9d (RCA) (e.g., see Y. Sato, xe2x80x9cA Method of Self-Recovering Equalization for Multilevel Amplitude-Modulation Systems,xe2x80x9d IEEE Trans. Commun., pp. 679-682, June 1975; and U.S. Pat. No. 4,227,152, issued Oct. 7, 1980 to Godard); and the other technique is the so-called xe2x80x9cconstant modulus algorithmxe2x80x9d (CMA) (e.g., see D. N. Godard, xe2x80x9cSelf-Recovering Equalization and Carrier Tracking in Two-Dimensional Data Communications Systems,xe2x80x9d IEEE Trans. Commun., vol. 28, no. 11, pp. 1867-1875, November 1980; and N. K Jablon, xe2x80x9cJoint Blind Equalization, Carrier Recovery, and Timing Recovery for High-Order QAM Signal Constellationsxe2x80x9dIEEE Trans. Signal Processing, vol. 40, no. 6, pp. 1383-1398, 1992.) Further, U.S. Pat. No. 5,793,807, issued Aug. 11, 1998 to Werner et al., and entitled xe2x80x9cBlind Equalization,xe2x80x9d presents a new blind equalization techniquexe2x80x94the multimodulus algorithm (MMA)xe2x80x94as an alternative to the above-mentioned RCA and CMA approaches.
However, for all blind equalization approaches the most fundamental performance issue is the ability to achieve reliable initial convergencexe2x80x94else the adaptive filter may converge to a wrong solution such as the well-known xe2x80x9cdiagonal solution.xe2x80x9d
We have discovered a technique for use in blind equalization of an adaptive equalizer that reduces the rate of occurrence of a diagonal solution. In particular, and in accordance with the invention, a receiver performs blind equalization by using a tap updating algorithm that includes a phase-compensating term.
In an embodiment of the invention, a receiver comprises an adaptive filter having a two-filter structure. The receiver uses a modified form of the CMA algorithm to perform blind equalization. This modified form of CMA is referred to herein as phase compensating CMA (PC-CMA). PC-CMA includes a phase-compensating term in the tap updating algorithm of the equalizer such that convergence to a straight constellation is achieved without the need of a rotator.